Chemical vapor deposition apparatus activated by a microwave plasma

ABSTRACT

Apparatus for plasma activated chemical vapor deposition, the apparatus comprising a microwave-excited plasma reactor with a reaction enclosure (10), a microwave generator (20), a waveguide (21) providing non-resonant coupling, and insertion means (40-54) for inserting at least one flow of a predetermined gaseous mixture into the enclosure; the insertion means comprise, in order: transformation means (40-43) for transforming the state of a precursor of a material to be deposited to bring it to the gaseous state, feed means (41, 42) for feeding a vector gas suitable for being charged with the gaseous precursor to constitute the above-mentioned predetermined gaseous mixture; and injection means (18) for injecting the predetermined gaseous mixture into the enclosure (10) and comprising an externally frustoconical nozzle provided with an injection orifice situated at one end and shaped as a function of the injection orifice and of the column configuration of the plasma formed, said nozzle having means for heating and thermally insulating the gaseous mixture.

BACKGROUND OF THE INVENTION

The present invention relates to the field of chemical vapor depositionwith the assistance of a plasma, and more specifically, it relates to anapparatus for plasma activated chemical vapor deposition, the apparatuscomprising, disposed one after another:

a plurality of transformation means for transforming the state ofrespective precursors of a plurality of deposition materials to causeeach of them to pass from an initial state to the gaseous state;

a plurality of charging means for charging vector gases with respectivegaseous precursors, each precursor-charged vector gas constituting apredetermined gaseous mixture;

a plurality of transfer means for transferring said predeterminedgaseous mixtures to a plasma reactor having microwave excitationcomprising a reaction enclosure, a microwave generator, and at least onewaveguide interposed between said generator and the enclosure andproviding non-resonant coupling; and

injection means for injecting said predetermined gaseous mixtures intothe reaction enclosure.

Apparatuses of the type to which the invention applies are designed toform deposits of special materials, in particular of ceramics, onsubstrates. It is thus possible, in particular, to make thermal barrierswhich, when deposited on an appropriate protective coating (e.g. ofMCrAlY material), enable turbine blades to be made that are wellprotected thermally: the operating temperature of the metal portionsremains below the allowed limit for the base material and the protectivecoating, while the temperature of the gas at the inlet of the turbinecan be considerably hotter (50° C. to 100° C.), thereby correspondinglyincreasing the efficiency of the turbine.

At present, three methods are known for obtaining thermal barriers:

1. The most widely used technique is plasma spraying. Grains ofpartially-stabilized zirconia powder are inserted into a plasma jet;they melt therein and are accelerated so as to be projected at highspeed against a facing substrate. They solidify thereon rapidly and theyadhere thereto by mechanically engaging roughnesses previously formed onthe surface of the part, generally by sandblasting. The resultingcoatings have a flaky structure that results from the stacking up ofdroplets that have flattened and solidified in lens-shapes, withsolidification being accompanied by microcracking. The highestperformance deposits are constituted by a layer of zirconia partiallystabilized by 6% to 8% by weight of yttria deposited on an underlayer ofMCrAlY alloy (where M═Ni and/or Co and/or Fe), itself deposited byplasma spraying under a controlled atmosphere. Generally, the ceramiclayers obtained in this way are about 300 μm thick. This technique leadsto deposits having microcracks and that may be constituted by metastablephases, with deposition speeds being very high, of the order of 100μm/min. It should nevertheless be observed that the method isdirectional and that parts of complex shapes are difficult or evenimpossible to cover. Furthermore, the roughness of such deposits makesfinishing treatment necessary to achieve a surface state that isaerodynamically satisfactory.

2. The method of evaporation under electron bombardment makes use of anelectron beam emitted by a heated filament. The beam is accelerated byapplication of an electric field and it is directed by means of amagnetic field onto the material to be evaporated, in the present case abar of yttrium-containing zirconia. Under the effect of such electronbombardment, the species are evaporated and condensed on the substratethat is placed facing the source. The substrate is optionally biased andis preheated and/or heated during the deposition operation. The resultsobtained by implementing this method present certain advantages:

the roughness of the layer obtained in this way is better adapted toaerodynamic flow;

the column structure of the deposit improves its thermomechanicalbehavior;

it has higher resistance to erosion; and

the layer adheres better.

However, it should be observed that making a coating ofyttrium-containing zirconia at a high deposition speed (100 μm/h)requires high electrical power to evaporate the bar of refractory oxide.In addition, implementation of this technique requires considerableinvestment and a large amount of know-how. Furthermore, this method islikewise directional, i.e. parts that are complex can be difficult oreven impossible to coat.

3. The radiofrequency cathode sputtering method makes it possible todeposit thin layers of yttrium-containing zirconia at low depositionspeeds of the order of 1 μm/h. In this method, a material raised to anegative potential is subjected to bombardment by positive ions. Theatoms of the material are ejected in all directions and condensed, inparticular on the substrate placed facing it so as to have a depositformed thereon. Systems have been used to make deposits of insulatingmaterials (deposits of oxides in particular) with possible modifications(magnetron, spraying in a reactive atmosphere, etc.) in order toincrease deposition speeds (up to a few μm/h). With this technique, itis certainly easier to control the composition of deposits than it iswith the method of evaporation under electron bombardment; howeverdeposition speeds are much slower and, as in the two preceding methods,deposition takes place directionally.

An essential object of the invention is to provide a novel depositionapparatus which enables the respective advantages of chemical vapordeposition and of plasma assistance to be combined, and in particular: amethod which is non-directional, takes place at a lower temperature andat an increased deposition speed, and a deposit which has a structurethat is controlled.

SUMMARY

To this end, a plasma activated chemical vapor deposition apparatus asdefined in the above preamble is essentially characterized, when inaccordance with the invention, in that the transfer means fortransferring the respective predetermined gaseous mixtures areindependent of one another, and in that the injection means include anozzle of frustoconical external profile into which the above-mentionedindependent transfer means open out and which is provided with at leastone injection orifice, the end of said nozzle being shaped as a functionof the desired configuration for the jet of ionized gas, said nozzlebeing fitted with means for heating and for thermally insulating thepredetermined gaseous mixture.

DESCRIPTION OF PREFERRED EMBODIMENTS

If the precursor used is in the solid state or the liquid state, thenadvantageously provision is made for the transformation means fortransforming the state of the precursor to comprise at least one chamberfitted with means for adjusting temperature and/or pressure to transformthe precursor into the gaseous phase.

If the precursor is in the solid state, then it is desirable for thecharging means for charging a vector gas with gaseous precursor to beorganized so that the vector gas passes through the precursor in powderform.

The above-mentioned vector gas may either be neutral or else chemicallyreactive with the gaseous precursor.

Preferably, the transfer means for transferring the predeterminedgaseous mixture include at least one transfer tube that is heated and/orthermally insulated, and at least one stop valve that is heated and/orthermally insulated. When a plurality of different predetermined gaseousmixtures need to be injected into the enclosure, provision may be madefor a mixing chamber into which all of the respective transfer tubes forthe different predetermined gaseous mixtures open out and which isdisposed upstream from the injection orifice.

For injection proper into the reaction enclosure, the nozzle may bearranged in any manner appropriate to the looked-for results: it may beprovided with a single outlet orifice, or it may be provided with amultiplicity of outlet orifices defined by subdivision means (such as agrid, a trellis, or a perforated plate) so as to ensure maximumionization of the jet of gas in the form of a column bearing against thecarrier of the sample to be coated that is disposed in the enclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic overall view of the essential means implementedin the plasma activated vapor deposition apparatus of the invention;

FIG. 2 is a cross-section through the sample carrier fitted withindependent biasing and heating means, as used in the FIG. 1 apparatus;

FIG. 3 is a cross-section view through the means for injecting into thereaction enclosure of the FIG. 1 apparatus; and

FIGS. 4A, 4B, and 4C show different shapes that can be adopted for theinjection nozzle of the means of FIG. 3.

DETAILED DESCRIPTION OF THE EMBODIMENTS

With reference initially to FIG. 1, the microwave plasma activatedchemical vapor deposition apparatus comprises a metal enclosure 10, e.g.made of brass, having a case that is generally rectangular or circularin shape.

The enclosure 10 includes an internal cylindrical tube 12 that iscentered relative to the enclosure 10 and whose wall is made of amaterial that has low dielectric losses, such as quartz.

The tube 12 has a circular opening 14 at its upper end and a circularopening 15 at its lower end.

The top portion of the enclosure 10 includes a circular opening 16centered relative to the enclosure 10 and of a diameter substantiallyequal to the diameter of the opening 14 of the tube 12. The circularopening 16 is fully closed by a circular metal cover 17 made ofstainless steel, for example. The cover 17 is pierced at its center byan orifice allowing a tube 18 to pass therethrough, the end of the tubehaving a special geometrical shape (e.g. cylindrical, hemispherical, orfrustoconical) and opening out into the tube 12.

The circular opening 15 of the bottom portion of the tube 12communicates with the outside via another cylindrical opening 29 ofdiameter substantially equal to that of the opening 15 and formed in thecover of a metal box 30 made of stainless steel, for example, and onwhich the tube 12 stands.

The apparatus also includes a microwave generator 20 emitting at afrequency of about 2.45 GHz and at a power of 1.2 kW.

A waveguide 21 conveys the microwaves to the tube 12 by non-resonantcoupling in which the tube 12 does not dissipate the microwave energycommunicated thereto in the form of electromagnetic radiation when gasis present in said tube 12. The electromagnetic energy propagates alongthe waveguide 21 in its longitudinal direction (arrow 19), with theelectric field E extending perpendicularly to the direction 19.

A bidirectional coupler 25 is provided in the initial portion of thewaveguide 21. In its intermediate portion 23 of rectangular section, thewaveguide includes a plurality of penetrating adjustment screws 26enabling the impedance of the reactor to be matched so as to obtain goodefficiency in the transmission of microwaves towards the tube 12. Theterminal portion 24 of the waveguide becomes progressively thinner inthe direction parallel to the field E and wider in the directionperpendicular to the field E so as to achieve a right cross-section thatis flat, rectangular, and substantially equal to the width of theenclosure 10.

The intermediate portion of the enclosure 10 is subdivided into twohalf-enclosures 11 and 13 which are separated from each other by a space(or gap) of rectangular right cross-section equal to that of theterminal portion 24 of the waveguide. The space (or gap) formed in thisway permits non-resonant coupling of the waveguide with the tube 12.

On its side diametrically opposite from the microwave feed 21, the tubeis provided with non-resonant coupling. The non-resonant coupling isestablished by a piston-type short circuit 27 having a flat rectangularright cross-section substantially equal to that of the terminal portionof the waveguide 24. The position of the piston 27 is adjusted to definea desired microwave electric field in the tube 12.

Inside the enclosure 10, a second microwave short circuit 28 is providedthat is coupled to the tube 12. The second short circuit 28 isconstituted by an annular plate that is also of the piston type. Theplasma originates in the tube 12 and it may be confined to a greater orlesser extent by adjusting the height of the annular plate 28.

The box 30 is generally circular in shape and it contains the samplecarrier 31 which supports the part to be coated that is capable of beingrotated (the part is not shown but it occupies the position referenced32), with the sample carrier being placed coaxially inside the tube 12.The sample carrier 31 is movable in vertical translation, therebyenabling it to carry the part 32 out from or into the plasma that isproduced in the tube 12.

The tube 12 and the box 30 are evacuated by primary pump means 33 (e.g.using Roots type pumps delivering about 125 cubic meters per hour)associated with a pressure gauge 34 that has an inlet connected to thebox 30 and an outlet connected to a pressure controller 35. In responseto the pressure measured by the gauge 34, the pressure controller 35actuates a motor that opens or closes a valve 36 connected to theprimary pump means 33 for the purpose of controlling the pressure in theenclosure.

With reference now to FIGS. 1 and 2 together, for the purpose ofcleaning a surface prior to deposition, the apparatus of the inventionincludes DC generator means 37 suitable for establishing a DC biasbetween the gas flow inlet means 18 and the metal portion 31a, e.g. madeof copper, of the sample carrier 31. Sometimes it is also necessary toheat the sample during such cleaning and/or while the deposit proper isbeing made. To this end, the apparatus further includes independentheating means 38 constituted by a coil of resistance wire disposedinside an alumina shell 38a. The heating system is regulated by means ofan external unit 39 controlled by a probe placed near the position 32.

The connection established between the voltage and temperature controlunits 37 and 38 respectively with the biasing and heating elements 31aand 38 are provided via sealed passages 38b.

With reference now to FIGS. 1 and 3, the apparatus of the inventionincludes means enabling gas to be fed via the tube 18 into the tube 12.The system for inserting precursors of the elements constituting theintended deposit is made up of two main zones having the followingfunctions, respectively:

converting the precursor from its initial state to the gaseous state;and

conveying the precursor in the gaseous state into the reaction enclosure12.

The precursors of the elements that constitute the deposit may besolids, liquids, or gases. With solids, the precursors are placed inenclosures 40 in which they are transformed into gaseous form by theeffects of temperature and/or pressure. These enclosures 40 includeheating elements 49a and thermal insulation 50a, and they are connectedupstream via conventional flexible or rigid tubes 41 to gas cylinders 42that may contain either reagent gases or else inert gases. Control means43 such as flowmeters and flow rate controllers enable the gas flow ratein the tubes to be controlled and regulated. The vector gases travelpast the precursors (solid, liquid, or gaseous) and, downstream from theenclosures and once charged with gaseous precursors, they penetrate intotubes 44 adapted to conveying gaseous mixtures. The tubes 44 areconventional tubes, each being provided with a heater 49b and thermalinsulation 50b. In addition, the tubes 44 are fitted with stop valves 45that are heated 49a and thermally insulated 50a to enable the precursorto be isolated. It is thus possible at will either to return theprecursor to the air or to keep it under the atmosphere of vector gas,depending on the risk of damage.

With reference to FIGS. 4A to 4C, the various precursors, all in gaseousform, may optionally be mixed in a mixing chamber 51 provided for thispurpose in the injection nozzle 52, which nozzle is tapering in outsideshape and terminates the tube 18 where it opens out into the tube 12.

The nozzle 52 is provided with a single orifice 53 (FIG. 4A) or elsewith multiple orifices 54 (FIGS. 4B and 4C), that are smaller than thesection of the tube 18.

The terminal portion of the tube 18 has an original geometrical shape,i.e. a truncated cone into which the vector gases charged with gaseousprecursors for making the deposit, either via a single orifice 51a orvia a plurality of orifices which may be defined by a grid or a trellis51b, for example (FIG. 4B), or else by a perforated plate in the form ofa spherical cap 51c (FIG. 4C), which shape makes it possible to ensuremaximum ionization of the gas jet in the form of a column bearing on thesample carrier. The width of said column is a function of thegeometrical shape selected for the injection nozzle (it increases ongoing from a single orifice to multiple orifices).

By way of example, and with reference to FIGS. 1 and 3, the use of theapparatus of the invention is now described in the case of depositing azirconium oxide. The metallic precursor is in the form of powder 40d andit is contained in a filtering crucible 40a made of quartz which isitself received in an enclosure 40b made of stainless steel. Thisenclosure is placed in an enclosure 40 that enables the powder to beheated by means of a system of heating collars 49a. The temperature ofthe enclosure 40 is regulated by means of an external unit that iscontrolled by a probe 40c placed in the powder. The oxygen, a reagentgas, is itself conveyed by means of a conventional tube 41a.

Argon, an inert gas, is inserted into the enclosure 40a by means of thetube 41. The powder heated under reduced pressure is converted to theform of a gas. The inert gas becomes charged with the gaseous precursor,passes through the powder, and is inserted into the enclosure 12together with the oxygen via the tube 44. The tube 44 is heated by asystem of heating cords that are coiled 49b and thermally insulated 50b,and the entire assembly is held in the tube 18. Handles 48 fixed on thetube 18 enable said tubes to be moved vertically. When in the highposition, the stop valve 55 may be closed so as to isolate theprecursor. The cover 17 and all the elements attached thereto may bemoved in vertical translation by a lifting system 46.

The oxygen over argon flow rate ratio may vary in the range 2 to 10.Tests have been performed with pressures in the enclosure lying in therange 100 Pa to 1000 Pa. Coupling is adjusted by positioning the piston27 and the matching screws 26. It has been possible to obtain a layer ofcolumn structure monoclinic zirconia at an average deposition speed of100 μm/h on a substrate of MCrAlY alloy.

In general, the apparatus of the invention makes it possible to make anytype of ceramic coating insofar as precursors exist or can be producedand are transportable in gaseous form. The ceramics may be made up ofpure oxides (SiO₂, Al₂ O₃, . . . ), combinations of oxides (ZrO₂ & Y₂O₃, SiO₂ & GeO₂, . . . ), metalloids (C, B, Si, . . . ), combinations ofmetalloids (SiC, B₄ C, BN, . . . ), or indeed metalloid/metalcombinations (TiC, TiB₂, AlN, . . . ).

An application of the apparatus lies in making thermal barriers ofzirconia partially stabilized with yttria. Uses can be envisaged in thefield of turbines (fabrication of fixed and moving blades).

Other applications may relate in particular:

to insulators for microelectronics;

to solid electrolytes for high temperature fuel cells;

to mirrors for laser optics; and

to alumina/zirconia composites (depositing matrix material on fibers,strands, fabrics).

The apparatus can be used in general in the surface coating andtreatment industry. For example, mention may be made of its applicationto making cutting tools.

Naturally, and as can be seen from the above, the invention is notlimited in any way to the particular applications and embodiments thathave been considered more particularly; on the contrary, it extends toall variants.

We claim:
 1. Apparatus for plasma activated chemical vapor deposition,the apparatus comprising:a plurality of transformation means, each fortransforming the state of a respective precursor of a depositionmaterial to cause each of said respective precursors to pass from aninitial state of matter to a gaseous state to provide a plurality ofgaseous precursors; a plurality of charging means, each for chargingrespective vector gas with a respective one of said plurality of gaseousprecursors to form a plurality of precursor-charged gaseous mixtures; aplurality of transfer means, each for transferring a respective one ofsaid predetermined plurality of gaseous mixtures from a respectivecharging means to a plasma reactor having microwave excitationcomprising a reaction enclosure, a microwave generator, and at least onewaveguide interposed between said generator and the enclosure andproviding non-resonant coupling; and injection means for injecting saidplurality of gaseous mixtures into said reaction enclosure in the formof a jet of ionized gas; each of said plurality of transfer means fortransferring a respective one of said plurality of gaseous mixturesbeing independent of other of said plurality of transfer means, saidinjection means including a nozzle of frustoconical external profile,said nozzle having an inlet for receiving said plurality of gaseousmixtures from said plurality of transfer means and having at least oneinjection orifice outlet for injecting said jet of ionized gas into saidreaction enclosure, said nozzle being shaped as a function of a desiredconfiguration of said jet of ionized gas, said nozzle being fitted withheating means and with thermal insulation means.
 2. Deposition apparatusaccording to claim 1, wherein each of said plurality of transformationmeans comprise at least one chamber and means for adjusting temperatureand/or pressure within said chamber.
 3. Deposition apparatus accordingto claim 1, wherein each of said plurality of charging means is adaptedto pass a respective vector gas through a respective precursor in powderform.
 4. Deposition apparatus according to claim 1, wherein a respectivevector gas is inert relative to a respective precursor.
 5. Depositionapparatus according to claim 1, wherein a respective vector gas ischemically reactive with a respective precursor.
 6. Deposition apparatusaccording to claim 1, wherein at least one of said plurality of transfermeans comprises a transfer tube that is heated and/or thermallyinsulated, and at least one stop valve that is heated and/or thermallyinsulated.
 7. Deposition apparatus according to claim 1, wherein saidnozzle includes a mixing chamber for mixing said plurality of gaseousmixtures, said mixing chamber being located upstream of said injectionorifice outlet.
 8. Deposition apparatus according to claim 1, whereinsaid nozzle is provided with a single outlet orifice.
 9. Depositionapparatus according to claim 1, wherein said nozzle is provided with amultiplicity of outlet orifices defined by a subdivision means. 10.Deposition apparatus according to claim 9, wherein said subdivisionmeans comprises a grid or trellis.
 11. Deposition apparatus according toclaim 9, wherein said subdivision means comprise a perforated plate.